This application relates generally to computer disc drives and more particularly to disc drive read/write head positioning.
Disc drives enable users of modern computer systems to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive houses a number of circular, magnetic discs (such as one to ten) which are axially aligned and rotated by a spindle motor at a constant, high speed (such as 10,000 revolutions per minute). As the discs are rotated, an actuator assembly moves an array of read/write heads out over the surfaces of the discs to store and retrieve the data from tracks defined on the surfaces of the discs.
A closed loop digital servo system is typically used to control the position of the heads relative to the tracks. The servo system generates a position error signal (PES) indicative of the position of the heads from servo information that is written to the discs during the manufacturing of the disc drive. In response to the detected position, the servo system outputs current to an actuator motor (such as a voice coil motor, or VCM) utilized to pivot the actuator assembly, and hence the heads, across the disc surfaces.
It is a continuing trend in the disc drive industry to provide successive generations of disc drive products with ever increasing data storage capacities and data transfer rates. Because the amount of disc surface area available for the recording of data remains substantially constant (or even decreases as disc drive form factors become smaller), substantial advancements in areal recording densities, both in terms of the number of bits that can be recorded on each track as well as the number of tracks on each disc, are continually being made in order to facilitate such increases in data capacity.
Recent changes in disc drive recording head technology have made it advantageous to use separate elements for reading and writing. Using two elements allows one element to be designed to perform as an optimum reader and the other as an optimum writer, avoiding the tradeoffs associated with implementing both functions in a single element. A performance synergy can result if the two are designed to complement each other. For example, a writer may be designed to write a relatively wide path and the reader designed to have a narrower read width. Together, they yield a greater storage density for a given level of tracking precision by permitting the reader to practically weave across a data track without sacrificing signal strength. Unfortunately, using separate read and write elements introduces a positioning problem because the two elements are separated by an appreciable gap. The gap and the skew angle make it impossible to position both elements in-line over all tracks. Consequently, in disc drives using magnetoresistive read elements and inductive write elements, there are separate read and write positions.
The servo information used to define the tracks is typically written to the discs following drive assembly in the manufacturing process using a highly precise servo track writer. While the tracks are intended to be concentric, uncontrolled factors such as vibrational tolerances in the servo track writer, spindle resonances, misalignments of the discs and the like tend to introduce if errors in the location of the servo information recorded on the discs. Each track is thus typically not perfectly concentric, but rather exhibits certain random, repeatable variations that are sometimes referred to as repeatable runout, or RRO, with the RRO appearing as an error component of the PES.
While RRO has previously had a minimal impact upon the operation of the servo system, RRO has an increasingly adverse affect as higher track densities are achieved. Particularly, RRO can ultimately lead to an upper limit on achievable track densities, as RRO cuts into the available track misalignment budget and reduces the range over which the servo system can provide stable servo control.
Accordingly there is a need for a method of correcting radial position error of disc drive heads.
Against this backdrop the present invention has been developed. The present invention is a method of correcting radial position error by utilizing predetermined error correction values located at predetermined locations in advance of their associated servo fields. During manufacture of a disc drive, radial position compensation values are calculated. The radial position compensation values are written to zero-acceleration path (ZAP) fields on the disc. Each ZAP field is positioned before the servo fields whose position error signal(PES) they are designed to correct. As the read/write head flies along a track, it reads a ZAP field, and stores the PES correction data. Subsequently, the read/write head reads the corresponding servo field and uses the stored PES correction data to correct the PES.
The ZAP field is comprised of two or more correction data elements, a starting sample number byte, and an error correction code. The starting sample number refers to the starting servo field and to a location in a data table where to store the correction data. Each correction data corresponds to each of the next subsequent servo fields following the starting servo field.
Two types of ZAP fields are written on the disc: read ZAP fields and write ZAP fields. The disc drive uses the read ZAP fields to correct the PES during subsequent read operations and write ZAP fields to correct the PES during subsequent write operations. Read ZAP fields are written in line with the data track nulls, while write ZAP fields are written in line with the servo track nulls. This novel arrangement allows for the read head to gather the ZAP field information in the write position, without having to move from the write position to the read position and back to the write position in the middle of a write operation.
The invention can provide multiple chances for the read/write head to read the ZAP fields prior to reading an associated servo field. For each servo field one or more ZAP fields may apply. Redundancy is built into the error correction method when more than one ZAP field refers to each servo field. Forward redundancy ensures that the ZAP field data is read correctly.
During normal disc drive operation, a ZAP table is used to store ZAP field data. The ZAP table is stored in memory. Upon any repositioning of the recording head, the table is initialized to zero. When a valid ZAP field is read, its correction data is saved in the ZAP table at the memory locations corresponding to the servo fields with which the ZAP field is associated. When the associated servo fields are subsequently read, the stored associated ZAP field data is retrieved and used to correct error in the PES.
If, upon a read of a ZAP field, it is determined that there are errors that the error correction code(ECC) process is unable to correct, the ZAP correction data is not saved in the table. Since the table was previously initialized to zero, correction of servo fields associated with the invalid ZAP field will effectively be a correction factor of zero, or no correction at all. In the preferred redundant system, the latest valid ZAP field correction data is stored. Thus, whenever the ZAP field is valid, the data is stored, and this will mean writing over previously valid data. This is not a problem because the redundant correction data for adjacent ZAP fields is the same.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.